1. Field of the Invention
This invention generally relates to remote control systems, more particularly to the field of the remote addressable devices and, more particularly, to a transducer of this type having automatic calibration and digital compensation functions.
2. Description of the Related Art
Remote control systems particularly designed for the acquisition of a plural measurements in a widespread geographic area have been known for sometime. These systems generally comprise a data acquisition center unit provided with a computer and a plurality of remote addressable transducers, also already known and supplying the measured values to the center unit under control thereof in a sequence predetermined by a different address code assigned to each transducer.
These remote addressable transducers, referred to later for the sake of brevity as RAD transducers (remote addressable devices), comprise a sensor for the quantity to be measured, a measure acquisition and conversion circuitry, a clock generating circuit, an addressing/timing device for determining the time of energization of the transducer according to the associated address, a power supply for supplying the energization voltage to the above components, as well as a line for supplying a signal proportional to the quantity to be measured by the sensors. This signal is generally a frequency.
These RAD transducers exhibit the following features:
1) possibility of being connected in a plurality (up to 127) to the acquisition unit through a single pair of wires to which they are parallel connected; PA1 2) possibility of being power supplied and of transmitting the measures thereby detected to the acquisition unit through this single pair of wires; PA1 3) possibility of being connected in a distance up to ten kilometers by the acquisition unit if a standard telephonic loop is used as the single pair of wires; PA1 4) possibility of being marked, when parallel connected to a line, through an address (number 1 to 127) set thereon in a binary configuration of bridges; PA1 5) possibility of replying in a sequence determined by their address in intervals of two seconds from each other from the time in which the line is power supplied, the reply comprising a modulation of the adsorbed current carried out with a frequency proportional to the value of the measured quantity. PA1 a) component procurement, PA1 b) assembly of the RAD transducer or the portions thereof, PA1 c) detection of the operating parameters before the calibration in a climatic chamber, PA1 d) calibration of the circuit according to the detected data and possible assembly of the portions, and PA1 e) final test in a climatic chamber. PA1 a microcontrollor, PA1 memory means connected to the microcontroller for memorizing the compensation parameters during the calibration PA1 an analogue/digital converter connected between the sensor of the quantity to be measured and the microcontroller for converting the analogue measure output of the sensor to a digital value adapted to be processed by the microcontroller, PA1 a temperature sensor which converts the temperature value to a voltage value, PA1 an analogue/digital converter which converts the analogue temperature measure output of the sensor to a digital value adapted to be processed by the microcontroller, and PA1 frequency generating means for generating the signal to be transmitted on the line through the power supply circuit.
The RAD transducers designed to meet these requirements must have a very reduced current consumption. As a matter of fact, they have to be power supplied by lines having a length of several kilometers and therefore a resistance value of some Kohms. To avoid excessive voltage drops along the line, the consumption is to be limited to few milliamperes. Furthermore, they must be ready to transmit a stable measurement in a very reduced time from the energization. In fact, the RAD transducers connected to a line normally remain in a wait condition with very reduced consumption (few microampere). One of the transducers is activated every two seconds following a sequence determined by the addressing and remains active for a second and half. To permit a correct acquisition of the measurement, the signals transmitted by the RAD transducer must be already stable a few hundred milliseconds after the energization.
The typical requirements of this type of RAD transducer for telephonic applications require overall dimensions to be reduced in order to be housed in a joint of a telephonic cable and a very low cost. The RAD transducers of this type are produced using a conventional analogic circuitry, partially in the form of hybrid circuits and partially in the form of surface mounted circuit, with functional calibrations carried out on the resistances of the hybrid circuit by means of a laser. The reduced dimensions of the device make very difficult the assembly of calibrating potentiometers which, inter alia, would reduce the reliability thereof.
The manufacturing process of a conventional RAD transducer comprises the following steps:
This calibrating procedure is rather difficult because the RAD transducer is to be placed twice in a climatic chamber, all the circuitry and the necessary mechanical operations are to be twice repeated and each single RAD transducer is to be calibrated by hand according to the data obtained during the step c).
The present invention aims at obviating these and other drawbacks which will be apparent in the following description.